This invention pertains to a loudspeaker apparatus, and more particularly pertains to a subwoofer loudspeaker system which utilizes a passive radiator.
A common limitation of loudspeaker systems, particularly those intended for high quality stereo sound reproduction, is relatively inadequate low frequency performance. Of course, one possible solution for those who are not satisfied with the low frequency performance of their stereo loudspeakers is to replace them with larger (and more expensive) loudspeaker systems. Another possibility is the addition of what is known as subwoofer systems to existing loudspeaker systems. However, there are several problems that have adversely affected the performance and marketability of subwoofer systems.
One of the most serious problems affecting the saleability of a subwoofer is that two subwoofers must be used, one for each of the channels in a stereo high-fidelity system. This effectively doubles the price. Since very low frequency sound (i.e. below 150 Hz) tends to have no directional effects in a stereo system, the improvement in low frequency performance afforded by a single subwoofer would be adequate in most cases. However, the use of a single subwoofer to which the stereo channels were coupled would unbalance the stereo channels.
Several previous designs have attempted to combine the electrical signals from the two stereo channels so that a single center channel subwoofer can be used. This is somewhat difficult in view of the substantial electrical power and low frequencies involved. Usually, it is accomplished through a "matrix" type crossover which includes both the large inductors required for the low crossover frequency and the transformers required to mix the two channels. One example of this general kind of arrangement in which the signals on the channels are electrically summed to feed a single low frequency speaker, is shown in U.S. Pat. No. 3,637,938.
Such an approach entails several problems. It is expensive and requires common ground outputs from an amplifier. Further, it is electrically inefficient and has poor performance due to inevitable saturation of transformer cores. Moreover, it also offers no flexibility and is too complicated for most consumers.
Another prior approach has been simply to build two subwoofers into a single divided cabinet, with or without separate built-in crossovers. This obviously reduces the cabinet cost but is otherwise equal to the cost of two separate subwoofers, and would result in a very bulky cabinet.